Most logging tools measuring porosity, resistivity, radioactivity and so on can record only average values of sand and shale properties in thinly laminated reservoirs. Only logging tools with very small radii of investigation such as the dipmeter or high frequency dielectric tool have vertical resolutions compatible with thicknesses of separate beds. These shallow investigation devices can be used to delineate thin beds within laminated reservoirs, and to determine shale volume. Such data are relatively shallow. However, evaluation of properties such as porosity, resistivity, and water saturation of these thin beds at greater distances has to be accomplished using logs with degraded vertical resolution, in combination with the high resolution shale indicator. This is possible mainly because the laminated shaly sand reservoir is a two component system (strata or laminar layers of sand and shale), and a change in properties of each of these components within the vertical resolution of most tools is rather insignificant compared to the drastic differences in properties between two components (shale versus sand) and therefore changes in the composite properties of the laminated reservoir. Since a sand component property and the logs with degraded vertical resolution usually have similar frequency characteristics, the former can be adequately restored from the logs. Subsequently, the high frequency composite log or formation property can be reconstructed from the component data and a high frequency shale indicator.
Several different methods of computing high resolution deep resistivity and other laminated reservoir parameter appear in the literature, Laminated Sand Analysis, D. F. Allen, SPWLA 25 Logging Symposium, Jun. 10-13, 1984; Comparative Results of Quantitative Laminated Sand Shale Analysis in Gulf Coast Wells Using Maximum Diplog Microresistivity Information, T. H. Quinn and A. K. Sinha, SPWLA 26th Annual Logging Symposium, Jun. 17-20, 1985; and Taking Into Account The Conductivity Contribution of Shale Laminations When Evaluating Closely Interlaminated Sand-Shale Hydrocarbon Bearing Reservoirs, J. Raiga-Clemenceau, SPWLA 29th Annual Logging Symposium, Jun. 5-8, 1988. The present technique has some features in common with these prior publications, but the overall teaching hereof is unique.
In laminated reservoirs, even productive ones, the volume of shale can exceed the volume of sand, and thus the volume of shale computation can strongly impact all subsequent evaluations. Accurate determination of shale volume is therefore significant. This approach matches shale volumes computed from one of several integrated high resolution shale indicators and a density-neutron crossplot by automatically adjusting shale parameters. These parameters are used to compute volumes of shale at two levels of vertical resolution; one of density and neutron logs and a second being a high resolution shale indicator. All other properties are also computed at two levels of vertical resolution, the first at low vertical resolution to compute components from composite properties and, the second at the high resolution to compute composite properties from sand and shale components. If the computations at lower resolution require different vertical resolutions, the logs with the higher vertical resolution are integrated to lower vertical resolution. Most existing water saturation equations, including the Waxman-Smits model, evaluate reservoirs having only dispersed clay. In the technique described below, the Waxman-Smits model is used to compute water saturation in the laminated shaly sand reservoirs but its parameters are adapted and used accordingly.
The disclosed technique works best in areas where properties of the sand and shale components of the laminated reservoirs have low frequency character, i.e. where variations within the sand and shale components occur much more slowly than variations in the composite character of the beds shown by high resolution logs (and caused by changes in the distribution of the laminae). This pattern is common for reservoirs in which effective porosity depends mainly on volume and mode of clay distribution. The Gulf Coast and similar areas where such laminated productive reservoirs are widely developed are primary targets for the technique described below.